1. Field of the Invention
The present invention relates to a photoelectric conversion device to be used in image sensors employed in copying machines, facsimile machines, video cameras and video recorders, optical sensors represented by AE sensors and AF sensors of cameras and sensors for detecting positions of objects, more specifically, a photoelectric conversion device suited to detect a light such as a micro spot light.
2. Related Background Art
FIGS. 1A to 1C respectively show an example of a conventional photoelectric conversion device (sensor) and FIG. 1A shows a 2-dimensional sensor which has 16 sensor cells, as photoelectric conversion elements, in total, including four sensor cells per line and four sensor cells per row.
In this sensor, each of lines is selected in sequence from the above in the drawing by a vertical shift register VSR and four discrete signals are outputted by the horizontal shift register HSR in time series sequence to the output terminal OUT.
Signals of these sensor cells are outputted in sequence in combinations of line scanning and row scanning.
In an actual sensor, the number of sensor cells amounts to 100 to 100,000 and therefore any method for reducing the read time and the scan time from one cell is obviously limited.
On the other hand, those signals from the sensor cells are visible image signals in most cases. In the case of such visible image, bright signals may be concentrated only at an extremely small area in one frame as the flame of a match in darkness and the remaining area may be occupied by dark signals.
Even in such case, the signals of all cells of the conventional sensor have been outputted in time series sequence and accumulated in an external random access memory, then required image signal processing has been carried out.
On the other hand, in case of the AE sensor (photo sensor for automatic exposure control), the size of the cell is expanded to reduce the number of divided parts and a configuration for ensuring short scanning time is used.
FIG. 1B shows a sensor as described above and each cell (SS11 . . . SS22) has a larger light receiving area than the cell shown in FIG. 1A and the number of divisions is 4.
The sensor shown in FIG. 1B, however, cannot discriminate uniform irradiation of weak light onto the whole light receiving surface of the cell (ma 1) from irradiation of strong light only at a part of the light receiving surface of the cell (ma 2) and therefore it is difficult to apply this sensor to detection of a spot light onto a small area.
As described above, the sensor has required a long processing time or has malfunctioned in detection of a light (ma 2) shown in FIG. 1C.
For example, FIG. 2 is a plan view of a pixel of a conventional bipolar sensor. In FIG. 2, 51 is an emitter (serving as the main electrode area where signals based on accumulated carriers are outputted), 52 is an output line formed with AL or the like, 53 is a contact hole for connecting an emitter 51 and an output line 52, 54 is a base (serving as the control electrode area) where an optical charge is accumulated, 55 is a drive line formed with poly-Si or the like for sensor operation of the pixels, 56 is a capacitor Cox formed between the base 54 and the drive line 55, and 57 is a gate of a MOS transistor which is formed with the base of an adjacent pixel as a source and a drain, and comprises part of the drive line 55. 58 is a thick oxidized film for separation between pixels.
FIG. 3 is a sectional view as FIG. 2 is sectioned along line XX′ and FIG. 4 is a sectional view as FIG. 2 is sectioned along line YY′. In FIGS. 3 and 4, 59 is a thin oxidized film, 60 is a high density n+ layer provided to separate pixel signals in the YY′ direction, 61 is an n epitaxial layer, 62 is a collector (serving as the main electrode area), and 63 is an inter-layer insulation film for separating wires 52 and 55.
In addition, FIG. 5 is an equivalent circuit diagram of an area sensor which is formed with the above-described pixels arranged in 2-dimensional format.
In FIG. 5, S is a pixel of the sensor (equivalently comprising a bipolar transistor 31, a capacitor COX4 and a PMOS transistor 5), 1 is a vertical output line to be connected to the emitter of the pixel S, 6 is a MOS transistor for resetting the vertical output line 1, 7 is a terminal for applying pulses to the gate of MOS transistor, 8 is a horizontal drive line, 9 is a buffer MOS transistor for receiving the output of the vertical shift register and passing a sensor drive pulse, 10 is a terminal for applying the sensor drive pulse, 11 is a wire connected to the drains of PMOS transistors at the right and left ends, 12 is an emitter-follower circuit part for setting a source potential of the PMOS transistor 5 to refresh the pixel S, 13 is a PMOS transistor for setting the base potential of the emitter-follower 12, 14 is a power supply terminal connected to the drain terminal of the PMOS transistor 13, 15 is a terminal for applying pulses to the gate of the PMOS transistor 13, 18 is an accumulation capacitor for accumulating output signal from the pixel S, 19 is a MOS transistor for transferring output signals to the accumulation capacitor 18, 20 is a terminal for applying pulses to the gate of the MOS transistor for transfer, 21 is a horizontal output line, 22 is a MOS transistor for receiving an output of a horizontal shift register and transferring output signals to the horizontal output line 21, 50 is a MOS transistor for resetting the horizontal output line 21, 23 is a terminal for applying pulses to the gate of the MOS transistor 50, and 24 is an amplifier.
A 2-dimensional solid image pickup apparatus shown in FIG. 5 is such that all pixels are reset at once, and can be used in a still video camera and the like.
The operation of this image pickup apparatus is briefly described below.
First a low-level pulse is applied to the terminal 15 to set the PMOS transistor to ON and the output of the emitter-follower circuit part to a positive potential. The output terminal of this emitter-follower circuit part 12 is connected to the source of the PMOS transistor for the pixel S and, if the source potential is sufficiently high enough to turn on the PMOS transistor 5 as compared with the gate potential, holes are injected into the base of the bipolar transistor 31 for pixels (referred to as the “first reset” up to this point). Then the transistor 6 is set to ON and the vertical output line 1 is set to the GND level by applying a high-level pulse to the terminal 7.
Next a forward bias is formed between the base and the emitter of the bipolar transistor 31 by driving the vertical shift register in the above state and applying a reset pulse for the pixels to the terminal 10 to reset in sequence the pixels of each line and set the base of the bipolar transistor 31 for all pixels to a fixed potential and to the reverse bias (referred to as the “second reset” up to this point). After accumulation of photo carriers, a low-level pulse is applied to the terminal 7 to set the MOS transistor 6 to OFF, a read pulse is applied from the terminal 10 to each line selected according to the output of the vertical shift register, a forward bias is formed between the base and the emitter of the bipolar transistor 31, and the signal output of pixels for each line is accumulated in the accumulation capacitor 18 through the MOS transistor 19. The signal output accumulated in the accumulation capacitor 18 is transferred to the horizontal output line 21 through the MOS transistor 22 for transfer selected by the horizontal shift register and outputted through the amplifier 24.
In this case, the accumulation time (Ts) of the sensor is a time from the end of the second reset to application of the read pulse to the terminal 10. In the case of the 1-dimensional linear sensor, the maximum value of the signal from each sensor cell (peak signal) is detected and the accumulation time is controlled according to this maximum value and, in the case of the 2-dimensional area sensor, it is difficult to detect the peak signal because of the property of the circuit and therefore it is also difficult to obtain an appropriate signal level for the whole image. For detecting a position of a pixel which presents the maximum or maximal output on the light receiving surface, there has been a problem that information (or all pixel outputs) should be used and therefore the signal processing time would be longer and a memory would be required.